It is known in the industry to provide a portable gas fired appliance that doesn't require an external power supply. The appliance typically includes a mesh screen that covers a plenum in which the gas is oxidized. The plenum acts as a reflector to direct the heat toward the mesh screen and evenly distribute it over the surface thereof. A flame sensor is usually incorporated into the assembly to provide a shutoff if the appliance is no longer operating properly.
The flame sensor is a thermocouple device that is positioned to sense the flame or an infrared bandwidth spectrum of approximately 600.degree.. For example, a sensor is mounted outside the plenum and directed toward the flame so that as long as a flame condition is sensed, sufficient current is supplied to a coil of a solenoid shutoff valve that supplies gas to the appliance. When energized, the coil creates a magnetic force that is sufficient to overcome the biasing force of a spring that urges the valve member toward a normally closed position. Thus, if a flame out condition is sensed, insufficient current is supplied to the solenoid coil and the valve member moves to the closed position, shutting off the supply of gas to the appliance. These types of shutoff assemblies have met with commercial success and are widely used in the industry.
Another area of increasing concern associated with these appliances, however, relates to carbon monoxide levels. There is an ever increasing emphasis on monitoring carbon monoxide levels with these types of gas fired appliances, particularly where there is a desire to use the appliance indoors or in, for example, the camping environment. The existing flame sensor cannot serve as a carbon monoxide sensor. Moreover, existing flame sensors or oxygen depletion sensors are based on a pilot flame or require an external source of power. Additionally, these arrangements are sensitive to ambient conditions, such as wind, humidity, elevation, or motion, and the desired carbon monoxide sensor should be immune to ambient conditions. Thus, there is a need to provide a sensor that can be used to detect carbon monoxide levels without these associated drawbacks.